A detecting device that detects a failure of a vehicle lighting device is provided in a vehicle in which the vehicle lighting device is provided. For example, if the detecting device detects a current flowing through a plurality of light emitting diodes which are connected in series and the detected current becomes a predetermined value or less, it is determined that a failure such as disconnection occurs in at least a part of the plurality of the light emitting diodes. If it is determined that a failure occurs, for example, the detecting device causes a display for notifying the failure of the vehicle lighting device to be turned on in a meter panel.
In this case, a vehicle lighting device including an incandescent light bulb and a vehicle lighting device including a light emitting diode may be selectively used depending on a grade of a vehicle type or the like. On the other hand, in many cases, the same detecting device is used to detect the failure of the vehicle lighting device regardless of the grade of the vehicle type or the like. If the detecting device is also used, since the number of thresholds for determining the failure is one, there is a concern that erroneous detection occurs.
Here, a voltage (input voltage) applied to the vehicle lighting device fluctuates. If the input voltage decreases, the amount of light emitted from the plurality of the light emitting diodes decreases so that there is a concern that a total luminous flux of the vehicle lighting device is less than a specified value. In addition, if the threshold based on the current flowing through the incandescent light bulb is used for determining the failure, there is a concern that it is determined that the failure occurs even if there is no failure when the input voltage decreases.
Therefore, it is desired to develop a technique in which, when the input voltage decreases, it is possible to secure a required total luminous flux and to suppress erroneous detection related to the occurrence of the failure.